As discussed in U.S. Pat. NO. 4,918,824 (issued Apr. 24, 1990 to Farrar), one fundamental vehicle navigation requirement is the direction in which the vehicle is traveling, and for about the last 2000 years the most fundamental navigation instrument has been the magnetic compass. The electronic compass is a single example of a general class of instruments called magnetometers (magnetic-field sensors), and more specifically it is classified as a vector magnetometer. This is because the earth's magnetic field is a 3-dimensional vector and the purpose of a compass, for example, in a vehicle, is to determine the vehicle's direction (magnetic heading) relative to the two horizontal components of this 3-dimensional vector field.
While there have been many forms of magnetometers developed, many electronic compass systems use what is called a flux-gate magnetometer. The flux-gate vector magnetometer is based upon a core made of high permeability magnetic material, wherein an external applied (or "environmental") field such as the earth's field, normally external to the core and along its axis, is alternately pulled into and released from the core material. This is what is referred to as gating the external field. This is done by driving the core of high permeability magnetic material into two antiparallel saturation states at a relatively high frequency with a primary coil. The transition of the core is sensed inductively by a secondary coil. The signal from the secondary coil is then coupled to a lockin-type amplifier which produces a DC signal proportional to the applied or environmental field.
Analyses of the flux-gate mechanism have been reported by a number of authors, for example: D. I. Gordon, et al., IEEE Transactions On Magnetics, MAG 1, No. 4, December 1965, "Factors Affecting the Sensitivity of Gamma-Level Ring Core Magnetometers"; S. V. Marshall, IEEE Transactions On Magnetics, MAG 3, September, 1967, "An Analytic Model For the Flux-Gate Magnetometer"; F. Primdahl, IEEE Transactions On Magnetics, MAG 6, No. 2, June 1970, "The Gating Curves of Parallel and Orthogonal Flux-Gates"; J. R. Burger, IEEE Transactions On Magnetics, MAG 8, No. 4, December 1972, "The Theoretical Output of a Ring Core Flux-Gate Magnetometer"; and D. I. Gordon et al., IEEE Transactions on Magnetics, MAG 8, No. 1, March 1972, "Recent Advances in Flux-Gate Magnetometry".
As discussed in U.S. Pat. No. 5,199,178 (issued Apr. 6, 1993 to Tong, et al.), a magnetic flux-gate sensor of the ring-shaped core type is relatively good in detection sensibility. Its structure is simple and its subsequent signal-treating circuit is relatively simple. However, it has the following disadvantages due to the present manufacturing processes: First, it is difficult to fabricate a magnetic core with uniform cross section to ensure a constant magnetic flux.
Further, automatic winding of a uniform excitation coil on a ring-shaped core is difficult, and non-uniform coil winding will cause a magnetic azimuth error to occur.
Further, it is difficult to obtain a uniform winding distribution in the sensing coils. When this happens, the directions of the input axis of the sensing coils are displaced from the desired direction with the result that a magnetic azimuth error occurs.
Further, the sensor is too large in size for certain practical uses (for example, a prior sensor has a diameter of 10-50 mm, and a thickness of 3 mm).
Further, a permalloy core, if not protected and supported, is easily broken by vibration and/or shock. A number of solutions have been proposed to alleviate the above problems.
For many magnetic materials, the electrical resistance depends on the angle between the current vector and the magnetization vector. Since the magnetization vector can be rotated by an applied magnetic field, the resistance depends on the applied magnetic field. This property is known as anisotropic magnetoresistance ("AMR"). In all cases, the AMR change in resistance is identical for antiparallel magnetization states.
Recently, a class of layered magnetic materials were discovered which had larger changes in resistance when magnetic fields were applied compared to AMR materials. The term "giant magnetoresistance" ("GMR") is used to describe these materials because the changes in resistance (3-200%), are much larger than those for traditional AMR materials (0-2%). The term "spin-valve magnetoresistance" ("SVMR") is also used for the GMR phenomenon because the resistance is observed to depend on the magnetization directions of neighboring magnetic materials. Rotating the magnetization vector of one layer relative to a neighboring layer changes the total resistance, which is analogous to a valve in a water pipe.
A typical device that acts as a magnetic sensor using spin-valve magnetoresistance is illustrated prior art in FIG. 5A. The upper ferromagnet layer 302 is pinned by the antiferromagnet layer 301; the magnetization vector stays pointing in one direction to first approximation. The lower ferromagnet layer 303 is designed to rotate freely in response to applied fields (a soft-magnetization layer), and is separated from the upper ferromagnet layer 302 by spacer layer 319.Therefore, applied fields change the angle between the respective magnetization vectors of the two layers. Changing this angle has a large effect on the resistance of the total structure, typically on the order of 2-8%. The hysteresis loop shown in prior art FIG. 6B showing resistance versus applied magnetic field represents the switching of the soft layer 303.
This concept is described in U;S. Pat. Nos. 5,206,590 (issued Aug. 23, 1994 to Dieny, et al.) and 5,159,513 (issued Oct. 27, 1992 to Dieny, et al.). In the most commonly described embodiment, the relative angle is designed to sit at the most sensitive point of the response curve while the sensor is moved past magnetic media. An applied field rotates the magnetization of the soft layer and produces a change in resistance.
Both AMR and SVMR materials can thus be used to detect magnetic fields. However, when used as a sensor for fields smaller than the saturation field of the soft ferromagnetic layer H.sub.SAT, the magnetization state of the soft layer then depends on its magnetic history. Further, as the magnetization direction of an MR structure is changed, various noise mechanisms, collectively called Barkhausen noise, prevent a smooth, reproducible hysteresis curve from being followed.
Therefore, a need exists for a low-cost, easy-to-construct magnetometer for weak magnetic fields.